This writer's mature concept (5)
I believe the answer is:
image
'concept' is the definition.
(thesaurus)
'this writer's mature' is the wordplay.
'this writer's' becomes 'I'm' ('I am' referring to the writer of the clue).
'mature' becomes 'age' (synonyms).
'im'+'age'='IMAGE'
(Other definitions for image that I've seen before include "Visualisation" , "Visible impression" , "Picture; simile" , "Presentation" , "Metaphor; figure" .)